Where does all this biomass come from?

Leaf structure and why it matters


Leaves exchange gases via stomata


Active vs Passive stomatal control


Leaf gas exchange impacts global ecology


Vascular tissues in leaves are confined to veins



  • Transpiration is a by‐product of opening stomatal pores

  • H2O transport needs to be sufficient to keep stomata open

  • Venation represents investment in H2O supply

How leaf veins changed the world….


Photosynthesis is complicated….



What do plants do when they are hungry?


Photosynthesis is the basis for most life on Earth





  • Greek = “building with light”
  • Autotroph = “self nourishing”
  • Photoautotroph = “self nourishing with light”


  • Photosynthesis is an energy pathway
    • 2 linked reactions
    • light reaction + Calvin cycle

Chloroplasts: Where the magic happens


The nature of light



When light hits chlorophyll a few things can happen


When pigments absorb photons an election gets excited


Photosystems (2) convert light energy to chemical energy


Light reactions: make ATP + NADPH for Calvin cycle



The Calvin Benson cycle makes sugar


Calvin cycle is a 3-step process



  • 1. Fixation: CO2 reacts with 5C molecule (RUBP)
    • new 6C split into 2 acids (3C each)
    • catalyzed by Rubisco enzyme


  • 2. ATP and NADPH turn 3C acids into sugars
    • reduction of acids


  • 3. Some 3C sugars recycled to make RUBP (5C)
    • one 3C sugar leaves to make glucose
    • recycling needs ATP

Making sugars requires several cycles


The brillance of photosynthesis



  • Light energy to synthesize something…
    • is light limiting?


  • Carbon dioxide & H2O are everywhere
    • easily diffuse into plant
    • contain little chemical energy
    • non-toxic


  • Chemical energy made includes ATP
    • high energy
    • movable
    • recyclable

Calvin Benson won a Nobel prize, his TA won best practical joke